Richard wrote:
>>I need to know how to determine if the following fans will be adequate
>>for an 4cx1000/1500 amp. While I can find the cfm specs, I cannot
>>find the back pressure specs.
>
>Ordinary fans do not create enough air pressure to cool external anode
>tubes. A centrifugal blower makes more pressure. I like to mount the
>tube/socket in a cardboard box and measure air pressure with a
>manometer.
>
>
Agreed about the cardboard box mock-up, just to see if the blower is big
enough.
Broadening the answer, it is better to measure the cfm directly by
letting the exhaust air blow up a thin plastic bag, and timing how long
it takes (as recommended here by K6GT). I use a 55 litre swing-bin bag
which typically inflates from flat to full in a few seconds... don't
know what size they are in the USA.
Measuring cfm also allows you to use other cooling methods, which may be
better than the manufacturer's standard system of blowing in through the
input compartment, through the socket and out through the anode (aka the
missionary system).
Blowing into a sealed output compartment and letting the hot air out
through a chimney on the *top* of the tube has many advantages.
Typically 25-30% of the airflow is bled down through the socket and out
through an exhaust port in the input compartment.
This method has been very widely used for the last 30-40 years in VHF
amps, where the necessary sealed output compartment is standard
practice. Advantages are:
1. It circulates *cold* air all around the tank components, unlike the
normal system which circulates hot air.
2. The pressure drops upwards through the anode cooler and downwards
through the socket are now in parallel, not in series as in the normal
method, so the blower has to provide a lower pressure. That generally
means that the same blower will provide *much* more airflow through the
anode in this configuration, as well as cooling the base of the tube
very adequately.
An hour or so spent with cardboard boxes, duct tape and a plastic bag
will show very clearly which method is superior, and whether your blower
is big enough.
I wouldn't recommend every sucking hot air into the amplifier, or
through the blower.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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